Cooling system, electronic equipment, and method for cooling heating element

ABSTRACT

A cooling system includes a first cooling part to cool a connecting part of a heating element with a first coolant having an electrical insulating property, the connecting part providing electrical connection between the heating element and a board, and a second cooling part to cool another part of the heating element with a second coolant, said other part being different from the connecting part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation application of International Application No. PCT/JP2010/064197 filed on Aug. 23, 2010 and designating the United States, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein relate to a cooling system, electronic equipment, and a method for cooling a heating element.

BACKGROUND

In recent years and continuing, along with acceleration of the processing speeds of information processing systems (such as server systems or computer systems), high-performance semiconductor equipment has been advancing. As the performance and functions of semiconductor equipment are enhanced, the sizes of the semiconductor devices or chips used in the semiconductor equipment become large and the amount of heat produced is increasing. Accordingly, techniques for efficiently cooling semiconductor devices have also been developed.

FIG. 1A illustrates a technique for cooling semiconductor equipment, which technique is known as a spray cooling method for spraying a pressurized coolant 103 from a nozzle 105 onto a semiconductor device 121 or a package 120. See, for example, Patent Documents 1 and 2 listed below. FIG. 1B illustrates another technique for immersing a semiconductor device 121 in a dielectric liquid (coolant) 104 with a low boiling point. This technique is known as an immersion cooling (or ebullient cooling) method.

Both techniques make use of boiling and vaporization of the coolant 103 or 104 to transfer heat from the semiconductor device 121 which is a heat source generating a large amount of heat during operation. The coolants 103 and 104 are circulated by a pump 108 and heat is removed by a radiator 106.

A fan 107 is used to enhance the heat removal efficiency.

Still another known technique is forming an air curtain around the chip during the spray cooling process. See, for example, Patent Document 3 listed below. In this technique, a chip is held upside down and coolant is sprayed by the nozzle toward the chip from underneath the chip, while supplying air flow in the opposite direction to the spray to produce an air curtain. The air curtain prevents the coolant from flowing into undesirable areas other than the heat generating surfaces to be cooled.

However, the spray cooling method illustrated in FIG. 1A has a problem because it is undesirable to spray a water-based coolant 103 directly onto the connecting part 125 that ensures electrical connection between interconnections which are insulated from each other. Spraying the coolant so as to avoid the connecting part 125 will lead to limited cooling ability. In addition, when the amount of heat produced by the semiconductor device 121 is large, boiling bubbles are generated and the efficiency for cooling the semiconductor device 121 is degraded.

The immersion cooling method illustrated in FIG. 1B is advantageous because the connecting part 125 of the semiconductor device 121 immersed in the dielectric coolant 104 is cooled directly. However, this technique uses a dielectric coolant 104. If a fluorinated coolant such as chlorofluorocarbon is used, environmental burden increases.

Therefore, it is desired to provide a cooling system and electronic equipment with the cooling system that can cool heat generating elements such as semiconductor devices in an efficient and stable manner while reducing the environmental burden.

-   -   Patent Document 1: Japanese Patent Laid-open Publication No.         H05-160313     -   Patent Document 2: Japanese Patent Laid-open Publication No.         H05-136305     -   Patent Document 3: Japanese Patent Laid-open Publication No.         H01-025447

SUMMARY

According to an aspect of the embodiments, a cooling system is provided. The cooling system includes:

a first cooling part to cool a connecting part of a heating element with a first coolant having an electrical insulating property, the connecting part providing electrical connection between the heating element and a board, and

a second cooling part to cool another part of the heating element with a second coolant, said other part being different from the connecting part.

According to another aspect of the invention, electronic equipment is provided. The electronic equipment includes:

a semiconductor device having a connecting part for connecting the semiconductor device to a board;

a first cooling part to cool the connecting part of the semiconductor device with a first coolant having an insulating property; and

a second cooling part to cool another part of the semiconductor device with a second coolant, said other part being different from the connecting part.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive to the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram illustrating a cooling system of a conventional spray cooling type;

FIG. 1B is a schematic diagram illustrating a cooling system of a conventional immersion cooling type;

FIG. 2 is a schematic diagram illustrating electronic equipment with a cooling system according to the first embodiment;

FIG. 3 illustrates a board on which multiple semiconductor devices are mounted together with other modules in a schematic plan view and a side view;

FIG. 4 is a schematic diagram illustrating a cooling system for cooling the board of FIG. 3;

FIG. 5 is a schematic diagram illustrating an experimental model to measure the cooling effect of the cooling system according to the first embodiment;

FIG. 6A is a diagram illustrating the cooling effect of the cooling system of the first embodiment compared with a conventional spray cooling system;

FIG. 6B is a table illustrating the cooling effect of the cooling system of the first embodiment compared with the conventional spray cooling system;

FIG. 7 is a schematic diagram illustrating semiconductor equipment with a cooling system according to the second embodiment;

FIG. 8 is a schematic diagram illustrating an experimental model to measure the cooling effect of the cooling system according to the second embodiment;

FIG. 9A is a diagram illustrating the cooling effect of the cooling system of the second embodiment compared with the conventional spray cooling system; and

FIG. 9B is a table illustrating the cooling effect of the cooling system of the second embodiment compared with the conventional spray cooling system.

DESCRIPTION OF EMBODIMENTS

The embodiments of the present disclosure are explained below with reference to the appended drawings. In the drawings, those elements with the same structure or functions are denoted by the same numerical symbols and repetitive explanations are omitted. In the embodiments, a cooling system is used to cool a semiconductor package mounted on a board; however, the cooling system described in the embodiments is suitable for cooling of arbitrary heat generating devices such as electronic modules or those devices having a connecting part for providing external electrical connection.

In the embodiments, a dielectric coolant and a water-based coolant, which separate from each other into two layers, are used. An electrical connecting part that produces a large amount of heat is directly cooled with a dielectric coolant, and the remaining heat generating parts other than the electrical connecting part are cooled with a water-based coolant. This arrangement achieves efficient and stable cooling, while reducing the environmental burden. The cooling system making use of two-layer separation is applied not only to horizontal semiconductor equipment in which a semiconductor package and a board are placed in a horizontal plane, but also to vertical semiconductor equipment in which boards are inserted vertically in a rack.

In the first embodiment, a cooling system is applied to a horizontally arranged semiconductor device, and in the second embodiment a cooling system is applied to a vertically arranged semiconductor device. In this context, the horizontal arrangement is one in which a semiconductor device and a board are placed in a plane perpendicular to the direction of gravity, and the vertical arrangement is one in which the semiconductor device and the board are place in a plane parallel to the direction of gravity. In the description, semiconductor elements (or chips), semiconductor packages, semiconductor modules and so on may be collectively called “semiconductor devices”. Similarly, circuit boards, interposer boards, system boards and so on may be collectively called “boards”.

First Embodiment

FIG. 2 is a schematic diagram illustrating electronic equipment 10 with a cooling system according to the first embodiment. In the first embodiment, a horizontally arranged semiconductor package 20 with a board 30 placed in a plane perpendicular to the direction of gravity is cooled.

The semiconductor package 20 includes a circuit board or an interposer board 22 (which may be simply referred to as “board 22”), and a semiconductor chip 21 electrically connected to the board 22 via solder bumps 23. The semiconductor chip 21 and the board 22 are entirely sealed in a package. The semiconductor package 20 has a connecting part 24 for providing electric connection between the semiconductor package 20 and a board 30 such as a printed circuit board. Heat produced by the semiconductor chip 21 is transferred via the solder bumps 23 to the board 22, and further transferred to the board 30 via the connecting part 24. The connecting part 24 generates more heat than the other parts and it needs to be cooled in an efficient manner. Because the connecting part 24 has a function of providing electrical connection with the board 30, it is undesirable to cool the connecting part 24 using a water-based coolant.

To solve this issue, a dielectric coolant 14 and a water-based coolant 13, which separate from each other into two layers in a casing 11, are employed. The dielectric coolant 14 is used to cool a surface including the connecting part 24 of the semiconductor package 20 which produces more heat. The water-based coolant 13 is used to cool the remaining parts of the semiconductor package 20, other than the connecting part 24 or the surface including the connecting part 24. The semiconductor package 20 and the board 30 are placed in the air-tight casing 11, and the dielectric coolant 14 is supplied in the casing 11 so as to immerse the connecting part 24 and the side faces of the semiconductor package 20 in the dielectric coolant 14. The casing 11 is formed of any suitable material, including a metal, a resin, a ceramic, a glass, etc. In the embodiment, a metal (such as aluminum) with a high thermal conductivity is used. The dielectric coolant 14 is preferably a non-corrosive and chemically stable fluid with an electrical insulating property. The coolants satisfying the above-described condition include fluorinated inactive liquids (such as FC-72), fluorocarbon coolants, hydrochlorofluorocarbons (such as HFC-365mfc, HFE-7000), halogenated hydrocarbon coolants (such as pentane), and dielectric oil based coolants containing, for example, silicone oil.

On the other hand, water-based coolant 13 is supplied onto the rear face 26 or other parts different from the connecting part 24 of the semiconductor package 20. The water-based coolant 13 is for example, water or pure water, which is sprayed onto the rear face 26 of the semiconductor package 20 from the nozzle 15 positioned above the semiconductor package 20. The specific gravity or the relative density of the dielectric coolant 14 is greater than that of the water-based coolant 13. When FC-72, which is a fluorinated inactive liquid, is used, the specific gravity of the dielectric coolant 14 is 1.68. Making use of the density difference, the dielectric coolant 14 and the water-based coolant 13 are separated into two layers.

The water-based coolant 13 ejected from the nozzle 15 hits the rear face 26, spreads to the peripheral regions of the semiconductor package 20 while absorbing the heat from the semiconductor package 20, and diffuses toward the inner wall of the casing 11 on the low-temperature side. Since the dielectric coolant 14 with a greater density stays under the water-based coolant 13, the water-based coolant 13 is prevented from flowing into the connecting part 24. The temperature of the water-based coolant 13 increases due to the heat absorption from the rear face 26 of the semiconductor package 20. The heated water-based coolant 13 is drained out of the casing 11 by the pump 18 a, and cooled through heat exchange at external cooling means such as radiator 16 and a fan 17. The cooled water-based coolant 13 is supplied to the nozzle 15 by the pump 18 b. The pumps 18 a and 18 b, the external cooling means 16 and 17 and the nozzle 15 are connected by pipes 19 and form a circulating system for the water-based coolant 13. The water-based coolant 13 heated and let out from the casing 11 is circulated and supplied back to the casing 11 to cool a part other than the connecting part 24.

The dielectric coolant 14 staying at the bottom of the casing 11 is evaporated and becomes vapor due to the heat generated from the connecting part 24 of the semiconductor package 20. The vapor of the dielectric coolant 14 dissolves in the water-based coolant 13. If the temperature of the water-based coolant 13 is lower than the boiling point of the dielectric coolant 14, the vapor of the dielectric coolant 14 will condense into liquid upon contact with the water-based coolant 13, and the liquidized dielectric coolant 14 naturally circulates back to the lower-layer dielectric coolant 14. If fluorinated inactive liquid FC-72 is used as the dielectric coolant 14, the boiling point is 56° C. If the temperature of the water-based coolant 13 in the casing 11 is maintained below 56° C. through the circulation, the dielectric coolant 14 circulates by itself in the casing 1. The low-boiling point fluorinated liquid 14 is prevented from vaporizing because the water-based coolant 13 functions as a shield.

Although not illustrated in the figure, a second circulating system for mechanically circulating the dielectric coolant 14 may be provided to the system in addition to the (first) circulating system for circulating the water-based coolant 13.

FIG. 2 illustrates an example of a single semiconductor package 20 to be cooled for the purposes of simplification. However, the cooling system of FIG. 2 is applicable to cooling a multi-CPU system with multiple semiconductor packages 20 and electronic modules mounted on the system board 30.

FIG. 3 illustrates a plan view of a multi-CPU system board 30, together with a cross-sectional view taken along the A-A′ line of the plan view. CPUs (semiconductor packages) 20 a, 20 b, 20 c and 20 d and other modules 32 are mounted on the board 30. The modules 32 are, for example, memory modules, switches, power modules, and so on. These modules are collectively denoted as memory modules 32 for the simplification purposes. In operation, the semiconductor packages 20 a-20 d and the memory modules 32 produce heat. To cool the multi-CPU system board 30 using a horizontal-type cooling system, multiple nozzles 15 may be positioned above the respective semiconductor packages 20 and the memory modules 32 to supply the water-based coolant 13.

FIG. 4 is a schematic diagram illustrating semiconductor equipment 40 with a cooling system for cooling the multi-CPU system board 30. The board 30 on which semiconductor packages 20 a and 20 b and a memory module 32 are mounted is placed in the air-tight casing 11. Dielectric coolant 14 is put in the casing 11 so as to cover the side faces and the connecting parts 24 of the semiconductor packages 20 a and 20 b and the memory module 32. Nozzles 15 a, 15 b and 15 c are positioned above the semiconductor packages 20 a and 20 b and the memory module 32, respectively, to spray the water-based coolant 13 onto the rear faces 26 a, 26 b and 36 of the semiconductor packages 20 a, 20 b and the memory module 32. The connecting parts 24 of the semiconductor packages 20 a, 20 b and the connecting part (not illustrated) of the memory module 32 for connecting the memory module 32 to the board 30 are directly cooled by the dielectric coolant 14. The water-based coolant 13 is circulated in the pipes 19 that connect pumps 18 a and 18 b and the cooling means 16 and 17. The water-based coolant 13 from which the heat has been removed is supplied to the nozzles 15 a-15 c.

In the examples illustrated in FIG. 3 and FIG. 4, the semiconductor packages 20 a through 20 d mounted on the board 30 may have the same size. Alternatively, semiconductor packages of different sizes may be mounted on the board 30. In the latter case, the semiconductor packages are cooled in the same manner as illustrated in FIG. 4. In still another alterative, a single nozzle 15 may be provided above the semiconductor packages 20 and the module 32. In this case, the spray direction of the nozzle 15 is regulated so as to cool the multiple semiconductor packages 20 and the module 32 evenly.

As has been described above, the first embodiment makes use of the difference in specific gravity between the dielectric coolant 14 and the water-based coolant 13 to separate the two fluids into two layers. The dielectric coolant 14 is used to cool the connecting part 24 of the semiconductor package 20 to ensure electric connection, and the water-based coolant 13 is used as the major cooling medium to cool the remaining parts such as the rear face 26 (positioned opposite to the connecting part 24) and to remove heat from the surroundings. The semiconductor device 21 or the semiconductor package 20 is cooled efficiently and stably, while reducing the environmental burden. Because the entirety of the semiconductor package 20 is immersed in the coolant, the semiconductor package 20 does not make contact with the external air. This arrangement is free from dew condensation, and migration at the connecting part 24 is prevented.

FIG. 5 is a schematic diagram illustrating an experimental model used to verify the effect of the first embodiment. A CPU (CORE 2 QUAD 3 GHz) 20 a manufactured by Intel Corporation and a peripheral component 32 are arranged as heating elements to be cooled. FC-72 which is a fluorinated inactive liquid is used as the dielectric coolant 14 in the experiment, and water is used as the water-based coolant 13. The heating elements are cooled making use of two-layer separation. The connecting part 24 of the CPU 20 a is immersed in the dielectric coolant 14 with greater relative density (specific gravity). The water-based coolant 13 with less relative density (specific gravity) is circulated by the pump 18 at a flow rate of 3 liter per minute. The water-based coolant 13 is subjected to heat exchange at a radiation amount of 80 W/h by the radiator 16, and then supplied to the nozzle 15. At CPU utilization of 100%, the internal temperature of the CPU 20 is monitored and measured. As a comparison example, the same CPU 20 and the peripheral component 32 are cooled by a spray cooling method illustrated in FIG. 1A using only the water-based coolant 13, and the internal CPU temperature is measured at CPU utilization of 100%.

FIG. 6A is a graph illustrating the experimental result, and FIG. 6B is a table in which the averaged CPU temperature and the equivalent heat generation of the experimental model are presented compared with those of the conventional model. As is understood from FIG. 6A and FIG. 6B, the CPU core temperature of the conventional model illustrated in

FIG. 1A exceeds 60° C. only a few minutes after the CPU utilization becomes 100%, and the averaged CPU temperature under the cooling environment is 61° C. On the contrary, the averaged CPU temperature at the 100% CPU utilization is 53° C. The equivalent heat generation of the conventional model is 180 W, while that of the experimental model of the first embodiment is 140 W. The structure of the first embodiment can achieve 40 W reduction in equivalent heat generation and 8° C. reduction in averaged CPU temperature. In the actual measurement result illustrated in FIG. 6A, the CPU core temperature varies at a certain amplitude. This is due to the influence of the operation of the CPU, and the stability of the cooling function of the system itself is guaranteed. By regulating the amount of heat radiation of the radiator, the flow rate of the pump, the layout of the nozzle(s) and the direction of the spray, the cooling ability can be further improved.

Second Embodiment

FIG. 7 is a schematic diagram illustrating electronic equipment 70 with a cooling system according to the second embodiment. In the second embodiment, a semiconductor package 20 is mounted on a vertical board 30 arranged in a vertical direction (along the direction of gravity). The structures of the semiconductor package 20 and the connecting part for providing the connection with the board 30 are the same as those illustrated in the first embodiment, and the explanation for them is omitted.

As in the first embodiment, the connecting part 24 of the semiconductor package 20 is directly cooled by the dielectric coolant 14, and other parts such as the rear face 26 (opposite to the connecting part 24 of the package) except for the connecting part 24 are cooled by the water-based coolant 13. To realize this arrangement in the vertical arrangement of the second embodiment, a first nozzle 75 is positioned above the vertically arranged board 30 with the semiconductor package 20 mounted, to supply the dielectric coolant 14 via the top edge of the board 30 to the connecting part 24. A second nozzle 15 is positioned so as to face the vertically arranged semiconductor package 20 to supply the water-based coolant 13 toward the rear face 26 of the semiconductor package 20.

The first nozzle 75 forms a curtain flow 76 so as to protect the end faces and the connecting part 24 of the semiconductor package 20 with the dielectric coolant 14. The dielectric coolant 14 is a chemically stable and non-corrosive fluid with an electrical insulating property as in the first embodiment. For example, fluorinated inactive liquids (such as FC-72), fluorocarbon coolants, hydrochlorofluorocarbons (such as HFC-365mfc, HFE-7000), halogenated hydrocarbon coolants (such as pentane), and dielectric oil based coolants containing silicone oil as the manor ingredient can be used as the dielectric coolant.

The second nozzle 15 sprays the water-based coolant 13 toward a part other than the connecting part 24, such as the rear face 26 opposite to the connecting part 24 of the semiconductor package 20. Since the connecting part 24 is protected by the curtain flow 76 of the dielectric coolant 14, the water-based coolant 13 is prevented from flowing into the connecting part 24. The dielectric coolant 14 and the water-based coolant 13 can be separated from each other in two layers along the direction of gravity.

The temperatures of the dielectric coolant 14 and the water-based coolant 13 rise through heat exchange with the semiconductor package 20. The heated dielectric coolant 14 and the water-based coolant 13 are collected at the bottom of the casing 11. When a fluorinated inactive liquid such as FC-72 is used as the dielectric coolant 14, the specific gravity is greater than that of the water-based coolant 13. Because the dielectric coolant 14 and the water-based coolant 13 flow down to the bottom of the casing 11, portions of the two liquids mix with each other at the bottom of the casing 11.

The heated dielectric coolant 14 and the water-based coolant 13 are let out from the bottom or the lower part of the casing 11 via the first pipe 19 a, and subjected to heat exchange at the radiator 16 and the fan 17. The coolants from which the heat has been removed by the heat exchange are supplied to the separation tank 79. In the separation tank 79, the dielectric coolant 14 and the water-based coolant 13 naturally separate into two layers because of the difference in the specific gravities. The dielectric coolant 14 separated from the water-based coolant 13 is supplied to the nozzle 75 via the pump 78 a and the second pipe 19 b. The water-based coolant 13 is supplied to the second nozzle 15 via the pump 78 b and the third pipe 19 c. Making use of the two-layer separation of the coolants, the heat-removed water-based coolant 13 and the dielectric coolant 14 can be circulated to the corresponding nozzles 15 and 75, respectively. With this arrangement, the vertically arranged semiconductor package 20 can be cooled efficiently. Another pump may be provided in the first pipe 19 a as necessary.

The structure of the second embodiment is applicable to a vertical arrangement of the multi-CPU system board 30 illustrated in FIG. 3. In this case, the first nozzles 75 may be provided corresponding to the respective columns of the semiconductor packages 20 to form a curtain flow for each of the columns. Alternatively, the number of the first nozzles 75 may be appropriately determined according to the size, the shape or the structure of the openings of the nozzles 75, the flow rate of the dielectric coolant 14 to be sprayed, or the size of the board 30.

FIG. 8 is a schematic diagram illustrating an experimental model used to verify the effect of the second embodiment. A CPU (CORE 2 QUAD 3 GHz) 20 a manufactured by Intel Corporation and a peripheral component 32 are arranged as heating elements to be cooled. FC-72 which is a fluorinated inactive liquid is used as the dielectric coolant 14 in the experiment, and water is used as the water-based coolant 13. The heating elements are cooled making use of two-layer separation. The dielectric coolant 14 and the water-based coolant 13 are let out from the bottom of the casing 11 and heat exchange is performed at a radiation amount of 80 W/h by the radiator 16. The heat-removed dielectric coolant 14 and the water-based coolant 13 are separated into two layers in the separation tank 79 such that each layer extends in the horizontal direction. The dielectric coolant 14 is supplied to the nozzle 75 by the pump 78 a, and the water-based coolant 13 is supplied to the nozzle 15 by the pump 78 b. At CPU utilization of 100%, the internal temperature of the CPU 20 is monitored and measured. As a comparison example, the same CPU 20 and the peripheral component 32 are cooled by a spray cooling method illustrated in FIG. 1A using only the water-based coolant 13, and the internal CPU temperature is measured at CPU utilization of 100%.

FIG. 9A is a graph illustrating the experimental result, and FIG. 9B is a table in which the averaged CPU temperature and the equivalent heat generation of the experimental model are presented compared with those of the conventional model. As is understood from FIG. 9A and FIG. 9B, the CPU core temperature of the conventional model illustrated in

FIG. 1A exceeds 60° C. only a few minutes after the CPU utilization becomes 100%, and the averaged CPU temperature under the cooling environment is 61° C. On the contrary, the averaged CPU temperature at the 100% CPU utilization is 49° C. The equivalent heat generation of the conventional model is 180 W, while that of the experimental model of the first embodiment is 120 W. The structure of the second embodiment can achieve 60 W reduction in equivalent heat generation and 12° C. reduction in averaged CPU temperature.

The structure of the second embodiment can realize a more efficient and more stable cooling system as compared to the first embodiment. This may be because the dielectric coolant 14 is supplied as the curtain flow to the connecting part 24 of the semiconductor package 20 (see FIG. 7). By constantly supplying heat-removed dielectric coolant 14 to the connecting part 24 with a large amount of heat generation, high cooling efficiency is achieved.

According to the disclosures, the following effects can be achieved.

-   (1) High Cooling Efficiency: By combining spray cooling using a     water-based coolant and local cooling using a dielectric coolant for     connecting parts, the entire surfaces of a semiconductor device can     be cooled directly by liquid cooling. -   (2) Large-Area Cooling: The entirety of the system board including     memories, switches, power modules, etc., can be cooled at once. -   (3) Reduced Environmental Burden: By employing water-cooling as the     major cooling means and limiting use of fluorinated coolant, high     cooling efficiency is realized while reducing the environmental     burden. -   (4) High Reliability: Because the board does not contact the     external air, condensation due to temperature difference is avoided     and migration is prevented from occurring. -   (5) Reduced Cost: Because the entirety of the board is cooled, it is     unnecessary to provide a thermal module for each of the CPUs.     Heaters for preventing dew condensation can be omitted, and the     number of components and power consumption can be reduced.

The present disclosures can be applied to a cooling system for cooling an arbitrary heating element, and to electronic equipment with a cooling system. For example, the arrangements of the disclosures can be applied to a rack server or computer in which a number of vertical system boards are arranged side by side or a number of horizontal system boards are stacked.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A cooling system comprising: a first cooling part to cool a connecting part of a heating element with a first coolant having an electrical insulating property, the connecting part providing electrical connection between the heating element and a board; and a second cooling part to cool another part of the heating element with a second coolant, said other part being different from the connecting part.
 2. The cooling system according to claim 1, wherein the first cooling part is configured to immerse the connecting part of the heating element in the first coolant, and the second cooling part is configured to supply the second coolant to said other part of the heating element.
 3. The cooling system according to claim 2, further comprising: a casing to accommodate the first coolant and the second coolant, wherein a specific gravity of the first coolant is greater than a specific gravity of the second coolant, and wherein the second cooling part has a first supply unit to supply the second coolant taken out of the casing to said other part of the heating element.
 4. The cooling system according to claim 3, further comprising: a circulator to circulate the second coolant that has absorbed heat from the heating element; and a heat release part provided on a path of the circulator to remove the heat from the second coolant, wherein the circulator supplies the heat-removed second coolant to the first supply unit.
 5. The cooling system according to claim 1, wherein the first cooling part is configured to supply the first coolant to the connecting part of the heating element; and the second cooling part is configured to supply the second coolant to said other part of the heating element.
 6. The cooling system according to claim 5, further comprising: a tank to accommodate the first coolant and the second coolant, wherein a specific gravity of the first coolant is greater than a specific gravity of the second coolant, and wherein the first cooling part has a second supply unit to supply the first coolant from the tank to the connecting part of the heating element, and the second cooling part has a third supply unit to supply the second coolant from the tank to said other part of the heating element.
 7. The cooling system according to claim 6, wherein the second supply unit forms a curtain flow of the first coolant to surround the connecting part.
 8. The cooling system according to claim 6, further comprising: a first pipe to supply the first coolant and the second coolant having been supplied to the heating element to the tank; a second pipe to supply the first coolant from the tank to the second supply unit; and a third pipe to supply the second coolant from the tank to the third supply unit.
 9. The cooling system according claim 1, wherein the first coolant includes one of fluorocarbon, halogenated hydrocarbon, and dielectric oil.
 10. The cooling system according to claim 1, wherein the second coolant contains water or pure water as a major ingredient.
 11. Electronic equipment, comprising: a semiconductor device having a connecting part for connecting the semiconductor device to a board; a first cooling part to cool the connecting part of the semiconductor device with a first coolant having an insulating property; and a second cooling part to cool another part of the semiconductor device with a second coolant, said other part being different from the connecting part.
 12. The electronic equipment according to claim 11, wherein the first cooling part is configured to immerse the connecting part of the semiconductor device in the first coolant, and the second cooling part is configured to supply the second coolant to said other part of the semiconductor device.
 13. The electronic equipment according to claim 12, further comprising: a casing to accommodate the first coolant and the second coolant, wherein a specific gravity of the first coolant is greater than that of the second coolant, and wherein the second cooling part include a first supply unit to supply the second coolant taken out of the casing to said other part of the semiconductor device.
 14. The electronic equipment according to claim 13, further comprising: a circulator to circulate the second coolant that has cooled the semiconductor device; and a heat radiator provided on the circulator and to remove heat from the second coolant, wherein the circulator supplies the heat-removed second coolant to the first supply unit.
 15. The electronic equipment according to claim 11, wherein the first cooling part is configured to supply the first coolant to the connecting part of the semiconductor device, and the second cooling part is configured to supply the second coolant to said other part of the semiconductor device.
 16. The electronic equipment according to claim 15, further comprising: a tank to accommodate the first coolant and the second coolant having been supplied to the semiconductor device, wherein a specific gravity of the first coolant is greater than that of the second coolant, wherein the first cooling part has a second supply unit to supply the first coolant taken out of the tank to the connecting part of the semiconductor device, and wherein the second cooling part has a third supply unit to supply the second coolant taken out of the tank to said other part of the semiconductor device.
 17. The electronic equipment according to claim 16, wherein the second supply unit forms a curtain flow of the first coolant to surround the connecting part of the semiconductor device.
 18. The electronic equipment according to claim 16, further comprising: a first pipe to supply the first coolant and the second coolant having been supplied to the semiconductor device to the tank; a second pipe to supply the first coolant from the tank to the second supply unit; and a third pipe to supply the second coolant from the tank to the third supply unit.
 19. The electronic equipment according to claim 11, wherein the first coolant one of fluorocarbon, halogenated hydrocarbon, and dielectric oil.
 20. A method for cooling a heating element comprising: cooling a connecting part of a heating element with a first coolant having an electrical insulating property; and cooling another part of the heating element with a second coolant, said other part being different from the connecting part. 